Field of the Invention
This invention relates to piston rings for reciprocating engines or compressors and to methods of their manufacture.
It will be appreciated that when a piston ring, particularly (but not exclusively) a compression ring having a single gap, of either a plain rectangular cross-section or any other known or convenient cross-section, is installed in an engine, the outer periphery of the ring should be circular and should have an outward resilient load so that it maintains sealing contact with the cylinder bore around the entire periphery. When the ring and bore are new the gap should be small or substantially closed. It is also desirable that as the cylinder bore and/or piston ring wear during operation, the resilient sealing contact between the ring and the bore should be maintained around the entire periphery (although it will be obvious that as the diameter of the piston ring increases by say 0.5% of the diameter, the gap will increase correspondingly). Moreover, to promote good sealing and minimise friction, it is desirable for the outward resilient load to be as uniform as possible around the periphery.
In the following specification and claims, the term `diameter` is used in relation to the inner and outer peripheries of a ring blank even though these peripheries of the ring blank may be precisely circular but may, prior to machining, be slightly oval or elliptical. Nevertheless these peripheries of the ring blank will be approximately circular and so the term `diameter` will, for convenience, be used. Thus, references in the specification and claims to the `diameter` of the ring blank are to be construed accordingly.
The term `closed` as used herein in relation to the condition of a ring blank having a gap cut therein is defined as a condition of the ring blank in which the gap is reduced, as compared with a position of the ring blank when the ring blank is unstressed by external forces, or in which the free ends are lightly in abutment with one another. In most cases, however, there is still a small gap between the free ends in the `closed` condition. The finished ring when contracted to fit into a cylinder bore may be in a `closed` condition, as defined.
The term `circular` as used herein in relation to the machining of a clamped ring blank or blanks is to be construed as meaning circular within the tolerances of which the means used for machining are capable.